The most prominent use of an equation of state is to correlate densities of gases and liquids to temperatures and pressures. One of the simplest equations of state for this purpose is the ideal gas law, which is roughly accurate for weakly polar gases at low pressures and moderate temperatures. However, this equation becomes increasingly inaccurate at higher pressures and lower temperatures, and fails to predict condensation from a gas to a liquid. Therefore, a number of more accurate equations of state have been developed for gases and liquids. At present, there is no single equation of state that accurately predicts the properties of all substances under all conditions.

Equations of state can also describe solids, including the transition of solids from one crystalline state to another.

In a practical context, equations of state are instrumental for PVT calculations in process engineering problems, such as petroleum gas/liquid equilibrium calculations. A successful PVT model based on a fitted equation of state can be helpful to determine the state of the flow regime, the parameters for handling the reservoir fluids, and pipe sizing.

Measurements of equation-of-state parameters, especially at high pressures, can be made using lasers.[2][3][4]

Boyle's Law was perhaps the first expression of an equation of state.[citation needed] In 1662, the Irish physicist and chemist Robert Boyle performed a series of experiments employing a J-shaped glass tube, which was sealed on one end. Mercury was added to the tube, trapping a fixed quantity of air in the short, sealed end of the tube. Then the volume of gas was measured as additional mercury was added to the tube. The pressure of the gas could be determined by the difference between the mercury level in the short end of the tube and that in the long, open end. Through these experiments, Boyle noted that the gas volume varied inversely with the pressure. In mathematical form, this can be stated as:

pV=constant.{\displaystyle pV=\mathrm {constant} .\,\!}

The above relationship has also been attributed to Edme Mariotte and is sometimes referred to as Mariotte's law. However, Mariotte's work was not published until 1676.

In 1787 the French physicist Jacques Charles found that oxygen, nitrogen, hydrogen, carbon dioxide, and air expand to roughly the same extent over the same 80-kelvin interval. Later, in 1802, Joseph Louis Gay-Lussac published results of similar experiments, indicating a linear relationship between volume and temperature:

In 1834, Émile Clapeyron combined Boyle's Law and Charles' law into the first statement of the ideal gas law. Initially, the law was formulated as pVm = R(TC + 267) (with temperature expressed in degrees Celsius), where R is the gas constant. However, later work revealed that the number should actually be closer to 273.2, and then the Celsius scale was defined with 0°C = 273.15K, giving:

For a given amount of substance contained in a system, the temperature, volume, and pressure are not independent quantities; they are connected by a relationship of the general form

f(p,V,T)=0{\displaystyle f(p,V,T)=0}

An equation used to model this relationship is called an equation of state. In the following sections major equations of state are described, and the variables used here are defined as follows. Any consistent set of units may be used, although SI units are preferred. Absolute temperature refers to use of the Kelvin (K) or Rankine (°R) temperature scales, with zero being absolute zero.

If the calorically perfect gas approximation is used, then the ideal gas law may also be expressed as follows

p=ρ(γ−1)e{\displaystyle p=\rho (\gamma -1)e}

where ρ{\displaystyle \rho } is the density, γ=Cp/Cv{\displaystyle \gamma =C_{p}/C_{v}} is the adiabatic index (ratio of specific heats), e=CvT{\displaystyle e=C_{v}T} is the internal energy per unit mass (the "specific internal energy"), Cv{\displaystyle C_{v}} is the specific heat at constant volume, and Cp{\displaystyle C_{p}} is the specific heat at constant pressure.

where Vm{\displaystyle V_{m}} is molar volume. The substance-specific constants a{\displaystyle a} and b{\displaystyle b} can be calculated from the critical propertiespc,Tc{\displaystyle p_{c},T_{c}} and Vc{\displaystyle V_{c}} (noting that Vc{\displaystyle V_{c}} is the molar volume at the critical point) as:

a=3pcVc2{\displaystyle a=3p_{c}\,V_{c}^{2}}

b=Vc3.{\displaystyle b={\frac {V_{c}}{3}}.}

Also written as

a=27(RTc)264pc{\displaystyle a={\frac {27(R\,T_{c})^{2}}{64p_{c}}}}

b=RTc8pc.{\displaystyle b={\frac {R\,T_{c}}{8p_{c}}}.}

Proposed in 1873, the van der Waals equation of state was one of the first to perform markedly better than the ideal gas law. In this landmark equation a{\displaystyle a} is called the attraction parameter and b{\displaystyle b} the repulsion parameter or the effective molecular volume. While the equation is definitely superior to the ideal gas law and does predict the formation of a liquid phase, the agreement with experimental data is limited for conditions where the liquid forms. While the van der Waals equation is commonly referenced in text-books and papers for historical reasons, it is now obsolete. Other modern equations of only slightly greater complexity are much more accurate.

The van der Waals equation may be considered as the ideal gas law, "improved" due to two independent reasons:

Molecules are thought as particles with volume, not material points. Thus Vm{\displaystyle V_{m}} cannot be too little, less than some constant. So we get (Vm−b{\displaystyle V_{m}-b}) instead of Vm{\displaystyle V_{m}}.

While ideal gas molecules do not interact, we consider molecules attracting others within a distance of several molecules' radii. It makes no effect inside the material, but surface molecules are attracted into the material from the surface. We see this as diminishing of pressure on the outer shell (which is used in the ideal gas law), so we write (p+{\displaystyle p+} something) instead of p{\displaystyle p}. To evaluate this ‘something’, let's examine an additional force acting on an element of gas surface. While the force acting on each surface molecule is ~ρ{\displaystyle \rho }, the force acting on the whole element is ~ρ2{\displaystyle \rho ^{2}}~1Vm2{\displaystyle {\frac {1}{V_{m}^{2}}}}.

With the reduced state variables, i.e. Vr=Vm/Vc, Pr=P/Pc and Tr=T/Tc, the reduced form of the Van der Waals equation can be formulated:

For Pr<1 and Tr<1, the system is in a state of vapor–liquid equilibrium. The reduced cubic equation of state yields in that case 3 solutions. The largest and the lowest solution are the gas and liquid reduced volume.

Introduced in 1949, the Redlich-Kwong equation of state was a considerable improvement over other equations of the time. It is still of interest primarily due to its relatively simple form. While superior to the van der Waals equation of state, it performs poorly with respect to the liquid phase and thus cannot be used for accurately calculating vapor–liquid equilibria. However, it can be used in conjunction with separate liquid-phase correlations for this purpose.

The Redlich-Kwong equation is adequate for calculation of gas phase properties when the ratio of the pressure to the critical pressure (reduced pressure) is less than about one-half of the ratio of the temperature to the critical temperature (reduced temperature):

In 1972 G. Soave[6] replaced the 1/√T term of the Redlich-Kwong equation with a function α(T,ω) involving the temperature and the acentric factor (the resulting equation is also known as the Soave-Redlich-Kwong equation of state; SRK EOS). The α function was devised to fit the vapor pressure data of hydrocarbons and the equation does fairly well for these materials.

Note especially that this replacement changes the definition of a slightly, as the Tc{\displaystyle T_{c}} is now to the second power.

where α{\displaystyle \alpha } and other parts of the SRK EOS is defined in the SRK EOS section.

A downside of the SRK EOS, and other cubic EOS, is that the liquid molar volume is significantly less accurate than the gas molar volume. Peneloux et alios (1982)[7] proposed a simple correction for this by introducing a volume translation

Vm,SRK=Vm+c{\displaystyle V_{m,{\text{SRK}}}=V_{m}+c}

where c{\displaystyle c} is an additional fluid component parameter that translate the molar volume slightly. On the liquid branch of the EOS, a small change in molar volume corresponds to a large change in pressure. On the gas branch of the EOS, a small change in molar volume corresponds to a much smaller change in pressure than for the liquid branch. Thus, the perturbation of the molar gas volume is small. Unfortunately, there are two versions that occur in science and industry.

In the first version only Vm,SRK{\displaystyle V_{m,{\text{SRK}}}} is translated,[8][9] and the EOS becomes

In the second version both Vm,SRK{\displaystyle V_{m,{\text{SRK}}}} and bSRK{\displaystyle b_{\text{SRK}}} are translated, or the translation of Vm,SRK{\displaystyle V_{m,{\text{SRK}}}} is followed by a renaming of the composite parameter b − c.[10] This gives

A nice feature with the volume translation method of Peneloux et alios (1982) is that it does not affect the vapor-liquid equilibrium calculations.[11] It should be noted that this method of volume translation can also be applied to other cubic EOSs if the c-parameter correlation is adjusted to match the selected EOS.

The model should provide reasonable accuracy near the critical point, particularly for calculations of the compressibility factor and liquid density.

The mixing rules should not employ more than a single binary interaction parameter, which should be independent of temperature, pressure and composition.

The equation should be applicable to all calculations of all fluid properties in natural gas processes.

For the most part the Peng–Robinson equation exhibits performance similar to the Soave equation, although it is generally superior in predicting the liquid densities of many materials, especially nonpolar ones.[13] The departure functions of the Peng–Robinson equation are given on a separate article.

A modification to the attraction term in the Peng–Robinson equation of state published by Stryjek and Vera in 1986 (PRSV) significantly improved the model's accuracy by introducing an adjustable pure component parameter and by modifying the polynomial fit of the acentric factor.[14]

where κ1{\displaystyle \,\kappa _{1}} is an adjustable pure component parameter. Stryjek and Vera published pure component parameters for many compounds of industrial interest in their original journal article. At reduced temperatures above 0.7, they recommend to set κ1=0{\displaystyle \,\kappa _{1}=0} and simply use κ=κ0{\displaystyle \kappa =\kappa _{0}}. For alcohols and water the value of κ1{\displaystyle \,\kappa _{1}} may be used up to the critical temperature and set to zero at higher temperatures.[14]

PRSV2 is particularly advantageous for VLE calculations. While PRSV1 does offer an advantage over the Peng–Robinson model for describing thermodynamic behavior, it is still not accurate enough, in general, for phase equilibrium calculations.[14] The highly non-linear behavior of phase-equilibrium calculation methods tends to amplify what would otherwise be acceptably small errors. It is therefore recommended that PRSV2 be used for equilibrium calculations when applying these models to a design. However, once the equilibrium state has been determined, the phase specific thermodynamic values at equilibrium may be determined by one of several simpler models with a reasonable degree of accuracy.[15]

One thing to note is that in the PRSV equation, the parameter fit is done in a particular temperature range which is usually below the critical temperature. Above the critical temperature, the PRSV alpha function tends to diverge and become arbitrarily large instead of tending towards 0. Because of this, alternate equations for alpha should be employed above the critical point. This is especially important for systems containing hydrogen which is often found at temperatures far above its critical point. Several alternate formulations have been proposed. Some well known ones are by Twu et all or by Mathias and Copeman.

The Elliott, Suresh, and Donohue (ESD) equation of state was proposed in 1990.[16] The equation seeks to correct a shortcoming in the Peng–Robinson EOS in that there was an inaccuracy in the van der Waals repulsive term. The EOS accounts for the effect of the shape of a non-polar molecule and can be extended to polymers with the addition of an extra term (not shown). The EOS itself was developed through modeling computer simulations and should capture the essential physics of the size, shape, and hydrogen bonding.

Noting the relationships between Boltzmann's constant and the Universal gas constant, and observing that the number of molecules can be expressed in terms of Avogadro's number and the molar mass, the reduced number density η{\displaystyle \eta } can be expressed in terms of the molar volume as

The Cubic-Plus-Association (CPA) equation of state combines the Soave-Redlich-Kwong equation with an association term from Wertheim theory.[17] The development of the equation began in 1995 as a research project that was funded by Shell, and in 1996 an article was published which presented the CPA equation of state.[17][18]

Although usually not the most convenient equation of state, the virial equation is important because it can be derived directly from statistical mechanics. This equation is also called the Kamerlingh Onnes equation. If appropriate assumptions are made about the mathematical form of intermolecular forces, theoretical expressions can be developed for each of the coefficients. A is the first virial coefficient, which has a constant value of 1 and makes the statement that when volume is large, all fluids behave like ideal gases. The second virial coefficient B corresponds to interactions between pairs of molecules, C to triplets, and so on. Accuracy can be increased indefinitely by considering higher order terms. The coefficients B, C, D, etc. are functions of temperature only.

One of the most accurate equations of state is that from Benedict-Webb-Rubin-Starling[19] shown next. It was very close to a virial equation of state. If the exponential term in it is expanded to two Taylor terms, a virial equation can be derived:

Note that in this virial equation, the fourth and fifth virial terms are zero. The second virial coefficient is monotonically decreasing as temperature is lowered. The third virial coefficient is monotonically increasing as temperature is lowered.

Statistical associating fluid theory (SAFT) equations of state use statistical mechanical methods (in particular perturbation theory) to describe the interactions between molecules in a system.[20] The idea of a SAFT equation of state was first proposed by Chapman et al. in 1989,[20] but since then many different SAFT equations of state have been proposed.[21] Often SAFT equations represent molecules as collections of spherical particles that interact with one another.[20] One popular SAFT equation represents molecules as chains composed of spherical segments (PC-SAFT).[22] In general, SAFT equations give more accurate results than traditional cubic equations of state, especially for systems containing liquids or solids.[23][24]

Multiparameter equations of state (MEOS) can be used to represent pure fluids with high accuracy, in both the liquid and gaseous states. MEOS's represent the Helmholtz function of the fluid as the sum of ideal gas and residual terms. Both terms are explicit in reduced temperature and reduced density - thus:

The reduced density and temperature are typically, though not always, the critical values for the pure fluid.

Other thermodynamic functions can be derived from the MEOS by using appropriate derivatives of the Helmholtz function; hence, because integration of the MEOS is not required, there are few restrictions as to the functional form of the ideal or residual terms. Typical MEOS use upwards of 50 fluid specific parameters, but are able to represent the fluid's properties with high accuracy. MEOS are available currently for about 50 of the most common industrial fluids including refrigerants. Mixture models also exist.

where e{\displaystyle e} is the internal energy per unit mass, γ{\displaystyle \gamma } is an empirically determined constant typically taken to be about 6.1, and p0{\displaystyle p^{0}} is another constant, representing the molecular attraction between water molecules. The magnitude of the correction is about 2 gigapascals (20,000 atmospheres).

The equation is stated in this form because the speed of sound in water is given by c2=γ(p+p0)/ρ{\displaystyle c^{2}=\gamma \left(p+p^{0}\right)/\rho }.

Thus water behaves as though it is an ideal gas that is already under about 20,000 atmospheres (2 GPa) pressure, and explains why water is commonly assumed to be incompressible: when the external pressure changes from 1 atmosphere to 2 atmospheres (100 kPa to 200 kPa), the water behaves as an ideal gas would when changing from 20,001 to 20,002 atmospheres (2000.1 MPa to 2000.2 MPa).

This equation mispredicts the specific heat capacity of water but few simple alternatives are available for severely nonisentropic processes such as strong shocks.

The ratio V=ρe/ρ{\displaystyle V=\rho _{e}/\rho } is defined by using ρe{\displaystyle \rho _{e}} = density of the explosive (solid part) and ρ{\displaystyle \rho } = density of the detonation products. The parameters A{\displaystyle A}, B{\displaystyle B}, R1{\displaystyle R_{1}}, R2{\displaystyle R_{2}} and ω{\displaystyle \omega } are given by several references.[25] In addition, the initial density (solid part) ρ0{\displaystyle \rho _{0}}, speed of detonation VD{\displaystyle V_{D}}, Chapman–Jouguet pressure PCJ{\displaystyle P_{CJ}} and the chemical energy of the explosive e0{\displaystyle e_{0}} are given in such references. These parameters are obtained by fitting the JWL-EOS to experimental results. Typical parameters for some explosives are listed in the table below.

where a0{\displaystyle a_{0}} = 0.02337 GPa.nm5. The total number of electrons Z{\displaystyle Z} in the initial volume V0{\displaystyle V_{0}} determines the Fermi gas pressure pFG0{\displaystyle p_{\text{FG0}}}, which provides for the correct behavior at extreme compression. So far there are no known "simple" solids that require higher order terms.

where β=K0{\displaystyle \beta =K_{0}} is the bulk modulus at equilibrium volume V0{\displaystyle V_{0}} and n=−K0′2{\displaystyle n=-{\frac {K'_{0}}{2}}} typically about −2 is often related to the Grüneisen parameter by n=−16−γG{\displaystyle n=-{\frac {1}{6}}-\gamma _{G}}